The field of the invention relates to clamps for lifting articles such as steel plates and particularly to such lifting clamps having a locking mechanism to retain the clamp jaws in either open or closed positions.
One example in the prior art of this type lifting clamp is disclosed in Renfroe U.S. Pat. No. 2,654,630 issued Oct. 6, 1953. The clamp itions by one of the jaws being movable and operated by a shackle connected to this movable jaw by a force multiplying in linkage.
In prior art constructions the lifting clamp body is formed by spaced side plates with a pivotally mounted locking plate between these side plates that is operated by a handle located outside the side plates. A heavy tension spring connects the locking plate to the linkage which extends between the shackle and the movable jaw. Rotation of the handle to the closed position for the jaws causes the tension spring to exert a heavy closing force on the movable jaw. This handle rotation also causes the spring to move over center to urge the pivot pin, connecting the locking plate to the handle toward a position relative to its normal disposition that creates resistance to operation of the locking handle.
Another type lifting clamp found in the field of the instant invention is exemplified by Davies U.S. Pat. No. 4,162,804 issued July 31, 1979. The lifting clamp of this patent possesses the generally described features mentioned above but provides an added safety advantage. In the abovedescribed lifting clamp there is the danger that in use the operating handle may strike an object while the steel plate article is being lifted. This could result in release of the handle to its open position, thereby creating the danger that the steel plate may become released from the clamp during the intended lifting operation. In solving this problem the Davies U.S. Pat. No. 4,162,804 incorporates an auxiliary spring biased locking pin which is engageable with the locking handle to effectively latch the handle against inadvertent movement from its position where the lifting clamp jaws are urged toward their closed position.
However, in the above prior art constructions, operating conditions or use environments may be encountered in which operation or release of the auxiliary lock means, like that disclosed in the Davies patent, may become difficult or at least awkward to achieve. With the structure of the auxiliary lock of the Davies type lifting clamp the locking pin must be depressed before the lock handle can be moved to its position for opening the lifting clamp jaws. This locking pin in its operative position while holding or latching the locking handle is nested in a notch of the handle. Whereas this nesting of the pin in the handle notch offers advantages in protecting the pin from accidental or inadvertant depression or dislodgement as would release the handle, its location nested in the handle notch can create difficulty in depressing the locking pin. This difficultly can be accentuated in smaller size lifting clamps and when the lifting clamp is in a location or use environment where the access area to the locking pin is restricted.